In a closed-loop emission control apparatus wherein an exhaust composition is sensed to control the air fuel ratio with the sensed concentration of the exhaust composition, control oscillation is inevitable because of the inherent delay time involved in the cylinder cycles. As a result of the oscillation, air-fuel ratios tend to deviate greatly from the desired point (stoichiometry) and the residence time of the mixture outside of the stoichiometric window may prolong. According to a statistical analysis in which air fuel ratios are sampled and their occurrences are plotted, the sampled values form a distribution over a wide range of mixtures. From the emission control standpoint it is desirable that the sampled values distribute within a narrow stoichiometric window since noxious compositions (NOx, HC and CO) are simultaneously chemically converted into harmless materials at a maximum efficiency when the mixture is controlled in the neighborhood of the stoichiometry.